Skip to main content

Know How – Expert Edition – Alternator Diagnosis


27 Oct 2021

Category: Articles, Downloads

With MATT MASTERS

Matt Masters is back for the next installation in his rotating electrics focused Expert Edition pieces. This time, he’ll be taking us through alternator diagnosis and what to look for in order to ensure you make the right call. For more on our alternators and the rest of the NAPA range, you can contact the NAPA helpline at 03333 136597 to talk to a member of our technical support team.

INTRODUCTION

When it comes to alternator faults, misdiagnosis is common among both experienced and inexperienced technicians. A misdiagnosed alternator can lead to costly replacements or even a warranty rejection, both of which can spell frustration for technicians. If you would like a better idea of how to accurately diagnose alternator problems, NAPA has you covered.

IGNITION WARNING LAMP

The ignition warning lamp can respond to an alternator fault in three distinct ways:

If the warning lamp does not illuminate:

  • The ignition warning lamp has failed/relay is faulty (if fitted)
  • The ignition switch is faulty
  • The engine management system is faulty (In ECU-controlled vehicles)
  • The alternator is faulty/battery is excessively discharged

If the warning lamp stays on:

  • The battery connections are faulty/loose/corroded
  • The alternator/battery is faulty
  • Communication with the charging control unit has failed

If the warning lamp glows:

  • The battery/alternator connections are faulty/loose/corroded
  • Excessive electrical load caused by accessories/poor connections
  • The alternator/battery is faulty

OTHER REASONS FOR FAILURE

Possible reasons for an alternator failure can include:

  • Worn drive end bearings causing excessive movement and heat
  • Worn slip ring end bearings causing inconsistent contact between the brushes and slip ring
  • Worn brushes resulting in a poor connection
  • Broken mounting brackets or worn bearing housings
  • A defective voltage regulator or rectifier pack
  • An overrunning alternator pulley that no longer free-wheels

If your battery goes flat, it could indicate a parasitic load draining charge even when the vehicle is off. Excessive load due to accessories, lamps or other electric devices remaining powered should also be investigated along with the alternator

DID YOU KNOW?

During NAPA warranty inspections, alternators are visually inspected, bench-tested and component tested if necessary. If no manufacturing defect is found, you will receive a report explaining our inability to credit your return.

Become a NAPA Stockist   Download PDF Guide

 

Previous post

Next post

View all posts

Know How - Expert Edition - Starter Motor Diagnosis

With MATT MASTERS Rounding out his spotlight series on rotating electrics, Matt Masters applies his...

Read More

Know How - Expert Edition - How Do Starter Motors Work?

With MATT MASTERS Matt Masters returns for another Expert Edition piece on the ins and...

Read More

Know How - Technical Bulletin - TB-NST-03

FITMENT CHALLENGES – WISHBONES AND BALL JOINTS AUDI TT 98-06 When replacing the lower ball...

Read More

Strictly Necessary

These cookies are required for our website to operate and include items such as whether or not to display this pop-up box or your session when logging in to the website. These cookies cannot be disabled.

Performance

We use 3rd party services such as Google Analytics to measure the performance of our website. This helps us tailor the site content to our visitors needs.

Functional

From time to time, we may use cookies to store key pieces of information to make our site easier for you to use. Examples of this are remembering selected form options to speed up future uses of them. These cookies are not necessary for the site to work, but may enhance the browsing experience.

Targeting

We may use advertising services that include tracking beacons to allow us to target our visitors with specific adverts on other platforms such as search or social media. These cookies are not required but may improve the services we offer and promote.

Change Settings

Cookies

Welcome. You can control how we use cookies and 3rd party services below

Change Settings Accept All Cookies
Learn how we use cookies